It is carried out in boreholes or wells to derive the permeability of moderately or poorly permeable soils and can be performed at either a constant or a variable head.
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If the test is performed at a constant head, a continuous flow of water is introduced into the well so that the level remains constant; in this case, the permeability coefficient k is obtained from the following expression:
k=Q/(π ·d·H) for geotechnical boreholes
k=Q/(600·A·h) for wells
Where:
k (cm/s) represents the permeability coefficient
Q (cm3/s) injected flow rate,
d (m) borehole diameter,
h is the constant water height above the piezometric level,
A is a coefficient depending on the length and diameter of the casing:
A = α·(1,032L + 30d)
L (m) is the length of the filtering zone and d (m) the diameter of the slotted pipe. The value of α is as follows:
α = 1 (for L ≥ 6 m)
α = 0,481 + 0,178L – 0,014L2 (for L < 6 m)
If the test is performed at a variable head, a small volume of water must be introduced into the borehole up to a known height, and subsequently, the water height is measured at successive times. In this case, the formula used is:
k=(d2·dh)/(A·hm·dt)
Where:
d (cm) is the borehole diameter,
dh (cm) is the drawdown of the water level in the interval dt (s),
hm is the average height of the water level in the interval dt.